Changes in the amount of thiobarbituric acid-reactive substances (TBARS), an index of the lipid peroxidation, were estimated in liver, heart, and the red and white components of gastrocnemius muscle of control (C) and ubiquinone-treated rats (T, 10 mg.kg-1 daily, for 4 weeks) at rest, and 3 and 24 h after running until the rats were exhausted. The activity of creatine kinase in serum and the level of the non-protein sulphhydryl groups (NP-SH) in the above organs were also determined. The running time until exhaustion was longer in T than in C [83.0 (SD 13.8) vs 72.2 (SD 14.8) min, P < 0.05]. The exercise resulted in a significant increase (P < 0.01) in the TBARS amounts in all the tissues obtained from C and in the liver and heart of T. The postexercise amounts of TBARS were significantly higher (P < 0.01) in C than in T. The resting amounts of NP-SH in the heart and liver were significantly higher (P < 0.05) in C compared to T. The exercise led to a decrease in the NP-SH content in the heart and liver (P < 0.05) of C but did not seem to affect the NP-SH contents in these tissues in T. No effect of ubiquinone treatment on postexercise increase in the serum creatine kinase concentration was found. It was concluded that in rats ubiquinone treatment markedly suppresses exercise-induced lipid peroxidation in such organs as liver, heart and gastrocnemius muscle.